Sputtering describes a number of physical techniques commonly used in, for example, the semiconductor industry for the deposition of thin films of various metals such as aluminum, aluminum alloys, refractory metal suicides, gold, copper, titanium-tungsten, tungsten, molybdenum, tantalum, indium-tin-oxide (ITO) and less commonly silicon dioxide and silicon on an item (a substrate), for example a wafer or glass plate being processed. In general, the techniques involve producing a gas plasma of ionized inert gas "particles" (atoms or molecules) by using an electrical field in an evacuated chamber. The ionized particles are then directed toward a "target" and collide with it. As a result of the collisions, free atoms are released from the surface of the target as atom sized projectiles, essentially converting the target material to its gas phase. Most of the free atoms which escape the target surface condense (the atomic sized projectiles lodge on the surface of the substrate at impact) and form (deposit) a thin film on the surface of the object (e.g. wafer, substrate) being processed, which is located a relatively short distance from the target.
One common sputtering technique is magnetron sputtering. When processing wafers using magnetron sputtering, a magnetic field is used to concentrate sputtering action in the region of the magnetic field so that sputtering occurs at a higher rate and at a lower process pressure. The target itself is electrically biased with respect to the wafer and chamber, and functions as a cathode. The magnetic field's influence on the ions is proportional to its distance from the front of the target. Optimally a target assembly (the target and its backing plate) is thin to allow the magnetic field to have the greatest influence.
In generating the gas plasma and creating ion streams impacting on the cathode, considerable energy is used. This energy must be dissipated to avoid melting or nearly melting the structures and components involved. A technique used for cooling sputtering target assemblies is to pass water or other cooling liquid through fixed internal passages of the sputtering target assembly.
An example is shown in the simplified perspective sketch of FIG. 1, a sputtering system designed for large rectangular substrates, which includes a relatively thin sputtering target assembly with internal cooling passages. (Details of the chamber and its operation are described in earlier U.S. patent applications of the inventors: U.S. Ser. No. 08/157,763 filed Nov. 24, 1993 and U.S. Ser. No. 08/236,715 filed Apr. 29, 1994, now hereby incorporated by reference herein.) The processing/sputtering chamber 30 encloses a dark space ring 31 surrounding a substrate 32 to be sputter deposited. The upper flange of the sputtering chamber 30 supports a lower insulating ring 33 supporting a sputtering target assembly 40. The target material on the sputtering target assembly is facing toward the substrate 32 to be sputtered. The target assembly is negatively biased relative to the substrate to effect the sputtering. Inlet cooling lines 36 and outlet cooling lines 37 connect to cooling passages in the sputtering target assembly 40 to cool the assembly during sputtering. The top of the sputtering target assembly 40 is enclosed by a top chamber 35 supported on the back of the sputtering target assembly by an upper insulating ring 34. As fully discussed in the references previously cited, the top chamber 35 can house a moveable magnetron in an evacuated top chamber. The top chamber can be evacuated so that its pressure approaches the pressure of the process chamber. The force exerted on the area of the target assembly due to differential pressure between the process chamber and the top chamber is then minimal and easily restrained by the thin sputtering target assembly 40.
A multi-layered sputtering target assembly 40, as shown in FIGS. 2 and 3, is typically assembled according to the above mentioned patent applications using a two step process. In one step, a target material 48 is solder bonded to the backing plate 50. In another step, a finned (or grooved) cover plate 52 is bonded to the back of the backing plate 50 using a structural epoxy based adhesive. The structural epoxy based adhesive is cured by putting it in position and raising the temperature of the pieces to be joined while at the same time applying a pressure to keep the parts in intimate contact throughout the heating cycle. The order in which the two steps are done is dependent on the melting temperature of the solder and the curing temperature of the structural epoxy. The higher temperature bonding process is done first so that the integrity of the first formed bond is not affected by the subsequent process.
The process and materials used in producing a structural epoxy bond generally create a good bond; however, the cooling fluid occasionally leaks due to imperfections in bonding thereby causing such sputtering target assemblies to be rejected. The factors affecting the structural epoxy bond integrity are 1) surface treatment of the pieces to be joined, 2) epoxy selection and curing procedure, and 3) mechanical fitting or mating of the surfaces being joined prior to adhesive cure.
Surface treatment removes mechanically weak or non-adherent surface film on the metal. For example, surface treatment may simply consist of mechanically abrading the surface to be bonded in order to obtain a "clean" metal surface. Or, for superior results, the procedure may involve a) degreasing, followed by b) an acid etch to remove any visible oxide film or scale, c) rinsing to remove all traces of the acid, d) a surface-conditioning step to deliberately form a corrosion film of controlled chemical composition and thickness which promotes primer adhesion, e) drying, and f) priming within an hour to seal the surface from atmospheric oxygen and moisture.
Epoxy selection is based on several factors including: type of carrier, strength of the adhesive, adhesion to the primed surface, curing temperature and pressure procedure, and ability of the adhesive to flow to create a leak-free joint.
Surface treatment, epoxy selection, and curing procedures are factors controlled by manufacturing rigor. However, good mechanical fitting or mating of the surfaces being joined is also required to achieve leak-free joints. Distortion and voids are introduced by the two-step soldering process presently used to join large areas (e.g. 643 mm.times.550 mm target material dimension) of a) dissimilar metals and/or b) non-uniformly heated or cooled similar metals. The present process includes the solder wetting of the two surfaces to be bonded. The target material is then heated and a pool of solder is created at the soldering location. The backing plate, also heated, is then slid into the pool of solder to avoid trapping the solder oxide that normally floats over the molten solder, and the weight of the piece and a light pressure cause the solder in the pool to spread out over the surfaces to be soldered and bring the two materials generally in close contact. The pieces are held aligned one to the other until the solder cools below its melting temperature and the two pieces are bonded.
For example, when solder bonding indium-tin-oxide (ITO) to a commercially pure titanium backing plate, during cooling from the soldering temperature (e.g., 156.degree. C. for pure indium solder) to ambient temperature, the differential thermal contraction of the soldered connection tends to cause bending of the pieces. The edges of the target material, being the first to cool, initially form a stronger bond than the higher temperature center of the target. As a result, the strong connection between pieces of the outer edge of the target material causes the center of the target material to buckle and lift from the backing plate at the center of the target, by as much as 0.125" (3.175 mm), as the target material and backing plate continue to contract at different rates. In the subsequent structural epoxy bond step (the finned cover 52 is bonded to this highly distorted target/backing plate assembly), mechanical fitting or mating of the surfaces being joined is difficult. Poor mating results in uneven bond thickness which can cause the cooling fluid to leak resulting in rejection of the sputtering target assembly.
In addition, if such a sputtering target assembly is not flattened, the non-parallelism between the target material and the substrate being sputtered creates non-uniform films on the substrate. Raised areas at the center of the target material may create a void behind the raised area, or the target material may fracture. Such voids change the thermal conductivity between the target and backing plate and the temperature distribution across its face. Since the distribution of sputtered material and the rate of sputtering of the target are directly dependent respectively on the target material distance from the substrate and on its temperature, variations in the gap (distance) between the target and substrate and in the temperature of the target material will also change the uniformity of target material sputter deposited on the substrate.
Since the object of large area sputtering chambers, as described above, includes uniform film thickness across the entire area of the substrate being sputter deposited, variations in film thickness due to variable properties in the target surface of the sputtering target assembly are a great impediment to improving processing efficiency and sputter depositing a uniform film thickness over the whole surface.